Also, a slow loading JS lib wasn't as much of an issue 5-7 years ago when jQuery was really in its heyday. Web pages were simpler and more information based back then and it was usually a requirement that they be useable without javascript. Since you are (should be) loading jQuery at the end of the page (or asynchronously) the page should render just fine without jQuery and probably would have been loaded by the time any user interaction would have happened.
Also, since users were on computers (mobile wasn't really a thing) and data usage wasn't a limiting factor (most plans were unlimited), loading extra data didn't really matter that much.
Yeah, jQuery had extra functions that you didn't need, but it didn't really hurt much at the time and there wasn't really a better option.